A single sideband modulator provides a means of translating low frequency baseband signals directly to radio frequency in a single stage. Such modulators, providing suppressed carrier and one or two of the sidebands, facilitates the transmission of intelligence with significantly increased gain over AM transmission.
A well known way of creating a single sideband signal is to split a low frequency signal carrying intelligence into two identical but 90.degree. phase shifted signals. A radio frequency carrier signal is also split into two separate signals, one having a 90.degree. phase shift relative to the other. One radio frequency and one low frequency component are combined in each of two balanced modulators, the output signals of the modulators being summed. This system acts to suppress the carrier signal, and to provide sidebands, one being balanced out (cancelled) and the other being increased in the combined output.
The low frequency baseband signals can be provided by a digital signal processor and digital-to-analog converters. Signals can be generated this way with precise control over phase and frequency. If the desired signal is digitally generated as two quadrature signals, it can be up-converted by the modulator, with a single local oscillator.
However it has been found that such modulators are limited by several factors:
1. Local oscillator (carrier frequency) breakthrough in the balanced modulators.
2. The phase split of the local oscillator to the two balanced modulators must be precisely 90.degree. . Real phase splitters have their characteristics changed over time, temperature and frequency. The result is incomplete cancellation of the carrier and one sideband.
3. The two, mutually 90.degree. phase shifted carrier signals, when combined, must have the same amplitude. If they do not, incomplete cancellation as noted above occurs. Typically the carrier breakthrough and image sideband levels at the output of the modulator have been found to be between -10 and -20 dBc without careful calibration of the components.